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This CMMG kit gives you the two hardest parts to source when you want to build a 16.1-inch AR carbine in 5.7x28mm: a matched barrel and a bolt carrier group that speak the same language. The magic here is CMMG's patented Radial Delayed Blowback system. Instead of relying on a heavy bolt and stiff spring like a straight blowback gun, the bolt rotates slightly on firing to hold things closed a hair longer, letting chamber pressure drop before the action unlocks. The result is noticeably less felt recoil, a lighter reciprocating mass, and a gun that still gives you last-round bolt hold open like a real AR should.
The barrel runs 16.1 inches of 4140 chrome moly steel with a 1:9 twist and a medium taper profile, capped off with a 1/2×28 threaded muzzle for your brake, flash hider, or 5.7-rated can. Everything wears a salt bath nitride finish inside and out, which is about as tough as it gets for corrosion resistance and easy cleanup. That carbine-length barrel wrings good velocity out of the little 5.7 round, which is where this cartridge really comes alive with flat trajectory and minimal recoil.
These components are made to drop into a Mk4 or mil-spec AR15 upper. On the lower side, you can run a dedicated CMMG Mk57 lower that takes FN Five-seveN style mags, or use a standard AR15 lower paired with CMMG 5.7 AR Conversion Magazines. One thing builders appreciate is that there's no gas block or gas tube to fuss with. You torque the barrel nut to spec, drop in the BCG, and you're most of the way there.
The 5.7x28mm has picked up a serious following as a fast, lightweight round with flat trajectory and next to no kick, and it's a blast at the range as well as a popular choice for lightweight home-defense builds. Whether you're converting an existing upper or starting a fresh 5.7 project, this kit is the reliable heart of the build, backed by CMMG's Lifetime Quality Guarantee.
You'll need a Mk4 or mil-spec AR15 upper receiver, a barrel nut and handguard, plus a charging handle. On the lower end, either run a CMMG Mk57 lower that accepts FN Five-seveN style magazines, or use a standard AR15 lower with CMMG 5.7 AR Conversion Magazines. There's no gas block or gas tube to deal with, so assembly is simpler than a typical AR barrel swap.
No, and that's important. Radial Delayed Blowback barrels use a unique barrel extension that requires the matching CMMG 5.7x28mm RDB bolt carrier group, which is exactly why this comes as a matched kit. Running a standard AR15 or mismatched BCG won't work and isn't safe.
It does. The bolt rotates slightly on firing to delay unlocking until chamber pressure drops, which lets CMMG ditch the heavy bolt and stiff buffer spring that straight blowback guns rely on. Reviewers consistently describe the felt recoil as being close to a .22, and the system keeps proper AR features like last-round bolt hold open.
The 16.1-inch length gives you a carbine that handles well and pulls solid velocity out of the little 5.7 cartridge, which is a fast, flat-shooting round with very little kick. It's a popular length for range fun, varmint work, and lightweight home-defense builds. If you want something shorter, CMMG also offers 5-inch and 8-inch versions of this kit.
| UPC | 810097502413 |
|---|---|
| Manufacturer | |
| Manufacturer Part Number | 57D0476 |
| Model | |
| Length | 16.10" |
| Product Type | Barrel |
| Barrel Length | 16.1" |
| Caliber/Gauge | 5.7 x 28 mm |
| Color | Black |
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